grep: reuse getrenamedfn() from scmutil

My motivation is to reduce uses of filectx.renamed(). Reusing
scmutil.getrenamedfn() also means that we get some caching of copy
information per file and revision. I don't think that matters for `hg
grep` (I doubt it speeds up significantly, and I doubt it wastes
significant memory), but I'm not sure.

Differential Revision: https://phab.mercurial-scm.org/D6022

mercurial/commands.py

@@ -2940,6 +2940,7 b' def grep(ui, repo, pattern, *pats, **opt'
     found = False
     follow = opts.get('follow')
 
+    getrenamed = scmutil.getrenamedfn(repo)
     def prep(ctx, fns):
         rev = ctx.rev()
         pctx = ctx.p1()
@@ -2953,13 +2954,10 b' def grep(ui, repo, pattern, *pats, **opt'
                 fnode = ctx.filenode(fn)
             except error.LookupError:
                 continue
+
             copy = None
             if follow:
-                try:
-                    copied = flog.renamed(fnode)
-                except error.WdirUnsupported:
-                    copied = ctx[fn].renamed()
-                copy = copied and copied[0]
+                copy = getrenamed(fn, rev)
                 if copy:
                     copies.setdefault(rev, {})[fn] = copy
                     if fn in skip: